During overhead throwing energy must
be absorbed and distributed across soft tissues (i.e., musculotendinous
structures, joint capsule/ligaments) within a limited range of motion to
minimize the risk of injuries. This is primarily accomplished by the
interaction between the nervous and musculoskeletal systems that influences
shoulder stiffness. Active stiffness, which combines passive (e.g., capsule,
ligament, and tendons) with dynamic components (e.g., muscles co-contraction
and reflex responses), is an adaptive process that can help maintain stability
and dissipate energy during overhead throwing. Therefore, the objective of this
study was to identify if healthy baseball players present with uni-lateral
differences in neuromuscular control, posterior capsule thickness, and
glenohumeral stiffness regulation, which could be related to repetitive
throwing.Electromyography (EMG)
activity was recorded from upper, middle, and lower trapezius as well as serratus
anterior from 24 healthy baseball players. The investigators also used fine
wire EMG to accurately measure activity of 3 rotator cuff muscles (i.e.,
supraspinatus, infraspinatus, and teres minor). Bilateral measurements for
active glenohumeral stiffness were performed with a customized Stiffness and
Proprioception Assessment Device that quickly internally rotated the shoulder
20 degrees, from a 90°/90° position with 0° of external rotation. During this
quick movement the player performed a standardized contraction of his external
rotators. The investigators also assessed bilateral posterior capsule thickness
using ultrasound scanning. EMG results showed that serratus anterior
preparatory EMG and peak activity were larger on the dominant arm compared to
the non-dominant arm. In contrast, preparatory EMG co-contraction (i.e.,
supraspinatus/infraspinatus ratio and lower trapezius/serratus anterior ratio)
and reactive EMG co-contraction (i.e., supraspinatus/teres minor ratio) on the
dominant arm were less than the non-dominant arm. In addition to EMG
differences, the dominant arm also had significantly greater glenohumeral
stiffness compared to the non-dominant arm. Active glenohumeral stiffness was
significantly correlated with posterior capsule thickness.

In overhead athletes the posterior
capsule and surrounding muscles interact to maximize functional performance and
maintain glenohumeral stability. The authors of this study demonstrated that
there was altered muscle activity in the dominant arm compared to non-dominant
arm. For example, they found greater teres minor activity in the dominant
shoulder, which may reflect improved neuromuscular control and stiffness
regulation. This finding may suggest that strengthening the shoulder external
rotators may help reduce the risk of shoulder injuries, but this should be
confirmed in future studies. In addition to altered muscle activation, the authors
reported greater active glenohumeral joint stiffness in the dominant arm
compared to the nondominant arm, which may be an adaptive response that is intended
to enhance energy absorption during the deceleration phase of over hand throwing.
Finally, the authors found a correlation between posterior capsule thickness
and active glenohumeral stiffness when both arms are combined.
It would be interesting to see if the relationship between active stiffness and
posterior capsule thickness persists when pitchers with shoulder symptoms are
examined. Perhaps the symptomatic population will have increased posterior
capsule thickness and abnormally high active stiffness. Future research in this
area may help us better understand the role of the posterior capsule in
shoulder pathology and provide further rationale for stretches like the sleeperstretch, but we’ll have to wait for these
studies. Do you believe maladaptation occurs in the shoulder among athletes
with shoulder injuries? Do your rehabilitation programs for posterior shoulder
injuries focus on the teres minor and increasing stiffness of the shoulder
joint?

Mark that is a great question. We actually did measure EMG onset time for each of the muscles. This was reported in the manuscript. We did not see any bilateral differences in onset time for any of the muscles. These athletes were healthy so it would be interesting examining symptomatic athletes and identify if these EMG onset times are delayed which could leave the shoulder vulnerable to instability during the throwing motion. Thanks for commenting!

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